Since the invention of semiconductor devices based on the bipolar technology, such as the bipolar junction transistor (BJT), strong efforts have been made to increase the power handling capability of these devices in order to extend their applications. The metal-oxide-semiconductor field-effect transistor (MOSFET) is a transistor used for amplifying or switching electronic signals. The MOSFET is a four-terminal device with source (S), gate (G), drain (D), and body (B) terminals. The MOSFET is by far the most common transistor in both digital and analog circuits, though the BJT was at one time much more common.
As the CMOS technology gained importance and process technology in the field of integrated circuits was surpassing the development of bipolar technology used for power devices, the power MOSFET was introduced. It is now possible to use the steady progress in CMOS technology for the development of improved power devices such as the power MOSFET. Power MOSFET may have superior performance compared to its bipolar counterpart. For example, the n-channel power MOSFET is operating with electron transport which is inherently faster than the combined electron and hole transport the BJTs rely on.
Power MOSFETs are known for a better switching speed compared to BJT power devices, and they require very little gate drive power because of the insulated gate. The main drawbacks of the power MOSFET are high on-resistance Rdson and the hot carrier injection (HCI) problem. Methods and apparatus are needed to improve the power MOSFET performance on the high on-resistance and the HCI problem.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.